Origins
The English word 'child' occupies a unique and somewhat mysterious position in Germanic etymology. It descends from Old English 'cild,' from Proto-Germanic *kilþam, a word whose original meaning was probably 'womb' or 'fetus' rather than 'young person.' Unlike most basic vocabulary items, 'child' has few clear cognates in the other Germanic languages and no secure Indo-European etymology.
In Old English, 'cild' meant specifically an infant or baby, not a young person in general. The broader sense of 'young human being up to puberty' developed during the Middle English period. Old English had several other words for young people at different stages: 'bearn' (child, son — cognate with 'born' and 'bairn,' still used in Scottish English), 'cnapa' (boy, → 'knave'), and 'mægden' (maiden, girl). Of these, only 'child' survived as the standard neutral term.
The most linguistically fascinating aspect of 'child' is its plural, 'children,' which represents one of the English language's only double plurals. Old English had several plural formations, and 'cild' took what is called the -r- plural (or -ru plural), becoming 'cildru.' This rare plural class also included 'lamb/lambru' (lambs) and 'egg/ǣgru' (eggs). As the -r- plural class became increasingly rare and unfamiliar during the Middle English period, speakers began to feel that 'childer' (the evolved form of 'cildru') did not sound sufficiently plural. They added the -en suffix — one of the common Middle English plural markers, as in 'oxen' and 'brethren' — producing 'children.' The result is a plural marked twice: once by the archaic -r- and once by the -en.
Old English Period
This double-marking is extraordinarily rare in English. The only parallel is the dialectal 'kine' for cows, from Old English 'cȳ' + the -n plural ending. In both cases, the double plural arose because speakers lost awareness of the first plural marker and applied a second one for clarity.
The word 'child' had a specialized meaning in medieval English that has largely been forgotten. In Middle English romances and chronicles, 'childe' (with the final -e) was a title for a young nobleman who had not yet been knighted. 'Childe Roland,' 'Childe Harold' (Byron's famous poem), and 'Childe Rowland' (referenced by Shakespeare) all use 'childe' in this technical sense of 'noble youth in training.' This meaning derives from the Old English practice of sending noble children to be raised in other households — a 'cild' in a great lord's house was a fostering of rank.
The word's phonological history shows one significant change. Old English 'cild' had a short 'i' vowel, pronounced /tʃild/ (the 'c' before 'i' was already /tʃ/ in late Old English). During the Middle English period, the vowel was lengthened before the consonant cluster -ld, changing to /tʃiːld/. This long vowel then underwent the Great Vowel Shift, producing the modern diphthong /aɪ/ in /tʃaɪld/. The same lengthening and shift occurred in 'mild,' 'wild,' 'find,' 'mind,' and 'kind' — all words where a short vowel was lengthened before -ld or -nd clusters.
Proto-Indo-European Roots
Outside English, the word's nearest relatives are found in Scandinavian dialects — Swedish dialectal 'kulle' (boy) and related forms — but the connection is debated. Most Germanic languages use descendants of *barną ('child,' literally 'the borne one') instead, as in German 'Kind' (which, confusingly, comes from a completely different root — Proto-Germanic *kinþą, meaning 'descendant,' related to 'kin').